Method and apparatus for rearing post-larvae shrimp

ABSTRACT

A method and apparatus for rearing post-larvae shrimp wherein the shrimp are retained for an initial post-larvae period of growth in a first shrimp rearing means including a first habitat means comprising a first plurality of stacked substrates having a first combined total surface area and wherein the shrimp are retained for at least one subsequent period of growth in at least one further shrimp rearing means including a second habitat means comprising a second plurality of stacked substrates having a second combined total surface area greater than the aforesaid first combined total surface area. In further aspects of the invention, unique filtration means and culling means are also disclosed.

This is a division of application Ser. No. 952,664 filed Oct. 19, 1978,now U.S. Pat. No. 4,285,298.

BACKGROUND OF THE INVENTION

This invention pertains to a method and apparatus for rearing shrimpand, in particular, to a method and apparatus for rearing post-larvaeshrimp under controlled conditions.

In recent years attempts have been made to rear post-larvae shrimp undercontrolled conditions. Rearing shrimp in this manner has been carriedout employing so-called intensive culture systems and methods.

Systems of this type have been proposed wherein the post-larvae shrimpare reared in a succession of units each being provided with habitatstructures for the shrimp. U.S. Pat. No. 3,658,034 issued on Apr. 25,1972 discloses one such system wherein the initial unit of the systemcomprises a tank provided with a habitat structure formed from aplurality of vertical substrates. Following this initial unit, are anumber of further units which include tanks of increasing size and eachof which is provided with a habitat structure formed from a number ofcylindrical enclosures each for housing an individual shrimp. In thissystem, the tanks are disposed below one another so the shrimp contentof a higher tank can be emptied into a lower tank when the shrimp in thehigher hank have undergone a desired degree of growth. Moreover, in thissystem, the water in each tank is continuously circulated to removecontaminants therefrom.

Further U.S. patents which disclose intensive culture systems whereinvertically or horizontally aligned substrates provide habitats forpost-larvae shrimp are as follows: U.S. Pat. No. 3,985,101 issued onJuly 2, 1975; U.S. Pat. No. 3,916,833 issued on Nov. 4, 1975; and U.S.Pat. No. 3,889,639 issued on June 17, 1975. In the last named patent,the intensive culture system disclosed comprises a plurality ofhorizontally arranged nets and a filtration system whose filter isbackwashed by drainage of some of the water of the system. Moreover, inthis system, light is directed through the central area of the nets toattract molting shrimp to such areas and thereby prevent these shrimpfrom being cannibalized by the remaining non-molting shrimp.

It is an object of the present invention to provide an improved systemand method for rearing shrimp under controlled conditions on acommerical scale.

It is a further object of the present invention to provide a system andmethod for rearing shrimp wherein use of the rearing volume is maximizedin a manner that does not contribute to shrimp mortality.

It is yet a further object of the present invention to provide a systemand method for rearing shrimp wherein filtration of the rearing mediumis carried out in an advantageous manner.

SUMMARY OF THE INVENTION

In accordance with the principles of the present invention, the aboveand other objectives are realized in a system and apparatus comprising afirst shrimp rearing unit for retaining shrimp for an initialpost-larvae shrimp growing period including a first habitat structureformed from a first plurality of stacked substrates having a firstcombined total surface area and at least one further shrimp rearing unitfor retaining shrimp for a subsequent shrimp growing period including asecond habitat structure formed from a second plurality of stackedsubstrates having a second combined total surface area greater than theaforesaid first combined total area.

More particularly, the first combined total surface area is selected sothat the unit area per shrimp of the first plurality of substrates issufficiently large to encourage and promote the growth of shrimp ofsizes encompassed by the first shrimp growing period. The secondcombined total surface area, in turn, is selected to be greater than thefirst by an amount which results in a unit area per shrimp of the secondplurality of substrates which is sufficiently large to encourage andpromote the growth of shrimp of sizes encompassed by the second shrimpgrowing period. Preferably, the total second surface area and the unitarea per shrimp of the second plurality of substrates should be from 25to 200 percent greater than the total first surface area and the unitarea per shrimp, respectively, of the first plurality of substrates.

By providing both the first and second rearing units with habitatstructures formed from stacked substrates, the volume of the units forrearing shrimp is maximized for both the initial and subsequentpost-larvae growing periods. Moreover, the use of stacked substrates forthe subsequent growth period is found to better promote shrimp growthand life as compared to systems using other types of habitat structuresfor this growth period.

In a further aspect of the invention, the system is additionallyprovided with a filtration system which accomplishes filtration of therearing units through the use of a single pump. The remainder of thefiltration system operates via gravity flow. The aforesaid filtrationsystem is additionally provided with uniquely constructed filtrationassemblies (i.e., particulate filters, bio-filter, carbon filters andfoam fractioners) for appropriately filtering the medium of the rearingunits.

The system and method of the invention also contemplate the use of anovel technique and apparatus for movement of the shrimp from one unitto the other and for eventually harvesting same.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and aspects of the present invention willbecome more apparent upon reading the following detailed description, inconjunction with the accompanying drawings in which:

FIG. 1 shows in schematic fashion an overall view of an intensiveculture unit embodying an apparatus and method in accordance with theprinciples of the present invention;

FIG. 2 illustrates, in partial cross section, a side view of theintensive culture unit of FIG. 1;

FIG. 3 illustrates, in perspective view, a habitat structure and supportassembly of a shrimp rearing unit of the intensive culture unit of FIG.1;

FIG. 4 illustrates, in cross section taken along line 4--4 in FIG. 3,one of the substrate units of the habitat structure of FIG. 3 loweredinto a rearing tank;

FIG. 5 shows a cross section of the substrate unit of FIG. 4 taken alongthe line 5--5 in FIG. 4;

FIG. 6 illustrates, in perspective view, the overall filtration systemof the intensive culture unit of FIG. 1;

FIG. 7 shows a cross section of the bio-filter, particulate filters andfoam fractionator of the filtration system of FIG. 6 taken along theline 7--7 of FIG. 6.

FIGS. 8 and 9 illustrate cross sections of the particulate filter andthe bio-filter of FIG. 7, taken along the line 8--8 of FIG. 7;

FIGS. 10 and 11 illustrate cross sections of the foam fractionator ofFIG. 7 taken along the lines 10--10 and 11--11 of FIG. 7; and

FIGS. 12-15 show various views of the culling apparatus of one of theshrimp rearing units of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an overall view of an intensive culture system orunit 1 for rearing post-larvae shrimp in accordance with the principlesof the present invention. The system includes a shrimp rearing area 2comprised of three adjacent shrimp rearing units 2A, 2B and 2C and afiltration area 3 adjacent the rearing unit 2C. The rearing units 2A, 2Band 2C comprise rearing tanks 4A, 4B and 4C whose walls are formed fromcinder blocks as are those of the filtration area 3. A further cinderblock area forms a walkway 5 around the rearing units and the filtrationarea. This walkway and the walls of the rearing tanks and filtrationarea support a frame structure 6 on whose exterior is placed a plasticroof 6A (See, FIGS. 2 and 3) so as to form a fully enclosed system.Advantageously, in the area of the rearing units the plastic of theaforesaid roof is clear so that the roof acts to couple solar energyinto the system for heating same.

The rearing tank 4A of the rearing unit 2A holds post-larvae shrimp foran initial stage or period of post-larvae growth, while the tank 4B ofthe unit 2B holds shrimp who have completed this initial period ofgrowth for a second growth stage. The tank 4C of the unit 2C, in turn,holds shrimp who have undergone the aforesaid second growth stage anduntil the shrimp reach maturity. As is apparent, the rearing tanksincrease in volume, in going from the tank 4A holding the shrimp who arein the initial post-larvae growth stage to the tank 4C holding theshrimp that reach maturity. In the present illustrative case, this isaccomplished by increasing the length of the tanks, while maintainingtheir widths constant. The purpose of this increased volume is to permitthe use in the rearing units of habitat structures comprised of stackedsubstrates which increase in total surface area in going from therearing tank 4A to the rearing tank 4C. This increased surface areaaccommodates the increase in size of the shrimp, thereby affordingsufficient habitat area for promoting growth, while maximizing thevolume of the tanks useable for rearing.

As shown in FIGS. 1 and 2, the rearing units 2A, 2B and 2C thus includehabitat structures 7A, 7B and 7C. The aforesaid habitat structures 7A,7B and 7C are advantageously all constructed from a basic substrate unit9, the larger area structures being provided with an increased number ofunits to provide the increased area.

In the present illustrative case, the habitat structure 7A includeseight substrate units, the habitat structure 7B, sixteen substrate unitsand the habitat structure 7C, forty substrate units. The substrate unitsof each habitat structure, in turn, are arranged in banks 11 formed ofone or more columns of substrate units (FIGS. 1, 2 and 3). Each suchbank is supported on a common rectangular frame 12 which permits thebank to be inserted and lifted from the tank of its respective rearingunit. As shown, the substrate units of the banks are tied to theirrespective frames so they hang therefrom upon insertion into the rearingtanks.

The substrate banks 11 are lowered and raised from their respectiverearing tanks via the lines 13 of individual pulley systems 14associated with the banks and supported on the frame 6. These pulleysystems, in turn, are activated by winches (not shown) also supported onthe frame. When lowered into the rearing tanks the banks 11 aremaintained at a predetermined height above the tank bottoms. Thisfacilitates feeding of the shrimp as well as filtration of the tanks andclearing of the tank bottom. The banks are also maintained below thesurface of the medium 15 in their respective tanks.

As shown in FIGS. 3 to 5, each of the substrate units 9 comprises athree dimensional open rectangular frame 21 formed of tubing members.Stretched across opposite sides of the frame are a plurality of parallelsubstrates 22 which, as shown, are formed of meshed screening. Thesehorizontally displaced vertically arranged stacked substrates or screens22 serve as the habitats for the shrimp who crawl up them. The spacingbetween the substrates 22 of each substrate unit 9 is dependent upon anumber of factors, such as, for example, the rearing unit in which thesubstrate unit is to be situated, the size of the shrimp in such rearingunit, etc. Typically, the spacing may be in the range of 2 to 3 inchesdepending on the aforesaid factors.

As above-mentioned, the increase in total surface area of the substratesof each habitat structure relative to that of the preceding structure isdependent upon the increased unit area per shrimp required to sustainand encourage growth of the shrimp in each particular rearing unitrelative to that required in the preceding unit. The unit area pershrimp for encouraging growth in a particular rearing unit, of course,will depend heavily on the growth stage associated with that unit, asrelatively larger growth stages will result in shrimp of relativelylarger increased size and will require a proportionally relativelylarger unit area per shrimp to encourage growth. The number of growthstages employed, in turn, is dependent upon the desire to limit thesystem and, hence, the growth stages to a minimum so as to preservecompactness and minimize labor, while at the same time affording asufficient number of growth stages so that the difference in size of theshirmp in each stage and, hence, in a given rearing unit tank, is notsuch as to permit a large degree of cannibalism of the smaller shrimp bythe larger shrimp.

With these conditions in mind, it has been found that the growth stagessubsequent to the initial stage may encompass an increased growth in arange from 25 to 200 percent before either the number of rearing unitsbecomes too large, or the difference in the size of the shrimp in eachstage results in excessive mortality due to cannibalism. This means thateach growth stage subsequent to the initial stage may encompass anincrease in size of from 25 to 200 percent which, in turn, means thatthe increase in total substrate area and, hence, unit area per shrimpfrom one rearing unit to a subsequent unit will also be approximately inthe range from 25 to 200 percent.

In one embodiment of the present invention, it has been found desirableto select the initial growth stage to encompass a 200 percent growthfrom post-larvae size and each of the subsequent growth stages a 100percent increase in growth relative to the preceding stage. Thus, withthis embodiment of the invention three growth stages are required forthe shrimp to increase in size from their initial post-larvae size ofapproximately one-half inch to their adulthood size of 6 inches. Inparticular, the initial stage covers the growth period from 1/2 to 3/2inches, the second stage from 3/2 to 3 inches and the final stage from 3inches to 6 inches. Furthermore, in this embodiment, the number ofrearing units is three, as depicted, and the substrate units of therearing unit holding the shrimp in the final stage have a total combinedsurface area which is 100 percent greater than that of the units holdingthe shrimp in the middle stage, the surface area of the latter substrateunits, in turn, being 100 percent greater than that of the units holdingthe shrimp in the initial stage. Additionally, in this embodiment, thenumber of substrate units per stage is as depicted in FIG. 1, and thefirst and second growth stages each cover approximately a 6 week periodand the final growth stage covers approximately a 3 month period.

In order to further facilitate rearing of the shrimp and to furtherprevent the cannibalistic tendencies of the shrimp from increasingmortality, each rearing unit 2A, 2B and 2C is further provided withmolting areas which act as sanctuaries for molting shrimp who have losttheir outer shell and, therefore, are prone to attack from non-moltingshrimp. As shown, these areas are provided by mesh netting sections 23which are attached to the frames 12 and hang therefrom above thesubstrate banks 11. When the banks 11 are lowered into their respectiverearing tanks these netting sections lie within the tank mediumimmediately above the substrate units, and, hence, are accessible tomolting shrimp whose tendency when molting is to seek shelter away fromthe other shrimp.

To further facilitate use of these molting platforms, the nettingsections 23 are provided with means for creating high and low spotsrelative to the substrate units. This is simply and easily realized byattaching leads 24 and floats 25 at alternate positions along thenetting. The low spots provide areas where the molting shrimp can attachthemselves to the nets and the high spot areas where the attacked shrimpcan crawl to isolate them further from the other shrimp. Additionalisolation is achieved by providing dim lighting in the area of thenetting sections 23, this being accomplished by light 26 attached to theframe 6 and directed at the sections.

In order to facilitate the transferring of the shrimp from the rearingunit 2A to the unit 2B and from the rearing unit 2B to the unit 2C andto facilitate the removal of adult shrimp from rearing unit 2C, asimilar culling apparatus is provided in each of the rearing tanks 4A,4B and 4C. This apparatus permits selection of the shrimp who havereached the desired stage of growth or development to be transferred toor removed from their respective rearing tank quickly and efficiently.More specifically, as illustrated in FIGS. 12 through 15, each tank isprovided with a slotted track 121 which extends substantially around theperiphery of its four walls and whose ends terminate adjacent anelongated opening 122 closed by a screen 123 in the wall of the tankadjacent the next tank. The track 121 provides a guide means for a netstructure 124 whose ends are moved in opposite directions so as toprovide an enclosed area 147 including the screened opening 122.

More specifically, the net structure 124 comprises two similar elongatedhollow bars 125 and 126 each of which is provided with a guide structure127 at its upper end. The guide structure comprises two horizontal rail128 and 129 which are crossed by two vertical rails 131 and 132. Theupper horizontal rail 129 carries at its opposite ends horizontallyoriented rollers 133 and 134. The lower horizontal rail 128 also carriesat its opposite ends rollers 135 and 136, these rollers being orientedvertically. Inboard of the rollers 135 and 136, the rail 128 supportstwo wheels 137 and 138 arranged with their axes vertical. Two furtherwheels 139 and 141 having horizontal axes are connected via bars 142 and143 to the lower ends of the vertical rails 131 and 132.

Inserted in each of the hollow bars 125 and 126 is a tubular plasticmember 144. The members 144 support opposite ends of a mesh net 145. Thenet 145, in turn, passes through vertical slots 146 in the bars 125 and126 and its draw strings are gathered together at a common point abovethe top end of one of the bars. The net 145 is further provided withleads 148 and floats 149 which maintain the net in a verticallystretched condition.

In operation, the net structure 124 is placed in a respective tank withthe wheels 137 and 138 of the guide structures 127 of the bars 125 and126 inserted in the track slot. The rollers 135 and 136 and the wheels139 and 141 of the guides, in turn, engage the upper and lower walls ofthe track to prevent tilting. The two bars 125 and 126 are then moved inopposite directions until each arrives at an end of the screened opening122 in the tank wall. During such movement, the wheels 133 and 134 ofthe guide structures ride on the inner wall of the tank to further guidethe net structure. With the two bars 125 and 126 adjacent the ends ofthe opening 122, the net 145 now forms the enclosed area 147, which areaincludes the opening 122 and surrounds substantially all the shrimp inthe tank. The draw strings of the net 145 are then pulled, allowingthose shrimp who are smaller than the openings in the net to escape. Theaforesaid openings are selected to be approximately equal in size (i.e.,about 10 percent smaller) than the size of shrimp who have undergone thestage of development associated with the particular rearing tank. As aresult, the shrimp who remain trapped in the localized area 147surrounding the opening 122 are those who have substantially undergonethe desired degree of growth. The screen 123 closing the opening is thenslid upward and the aforesaid trapped shrimp move through the opening.If the tank in which the net structure is placed is either one of therearing tanks 4A or 4B the shrimp are transferred to the subsequentrearing tank, either the tank 4B or 4C. If on the other hand, the tankis the last rearing tank 4C, then the shrimp enter a harvestingcontainer and are removed from the system.

As above-mentioned, the system of the invention is also provided with afiltration area 3 located adjacent the rearing unit 2C. This filtrationarea provides filtration for the rearing tanks of the rearing units andmaintains the medium 15 therein substantially contaminant free.

More particularly, as shown in FIGS. 1, 2 and 6, the filtration area 3comprises a tank 31 which is partitioned by walls into a bio-filterfiltration section 32, a foam fractionator filtration section 33, acarbon filter filtration section 34 and filtration inlet and outletsections 35 and 36. The tank 31 also supports a particulate filterfiltration assembly 37 which is situated above the bio-filter section 32and a pump 38 which is situated on the tank wall separating the foamfractionator and carbon filter sections 33 and 34. The filtration inletsection 35 borders an aperture 41 closed off by a screen 42 in the wallof the rearing tank 4C. Medium 15 from the rearing tank 4C, as well asthe medium from the rearing tanks 4A and 4B and flowing into the tank 4Cthrough the screened apertures 43 and 44 (see, FIG. 1), thus enters theinlet section 35 through the screen 42. A cylindrical conduit 45connects the intake of the pump 38 to the inlet section 35 and the pump38 raises the energy of the medium flowing into the inlet section to alevel or head sufficient to carry the medium, via gravity flow, throughall the filtration sections and back to the rearing tanks. Filtering isthus carried out using a single pump and gravity flow, therebyminimizing the energy requirements needed for operation.

More specifically, the medium entering the pump 38 is coupled to thepump output which feeds a manifold 46 which couples the medium tocylindrical inputs 47 of the particulate filter assembly 37. Theseinputs feed a plurality of particulate filters 100 forming the filterassembly 37 and supported above the bio-filter section 32 on the wallsof tank 31 forming same. The medium passes through the filter assembly37, descends down through the bio-filter section 32 and is collected inapertured pipes 48 at the bottom thereof. The pipes 48, in turn, lead toa collector pipe 49 which carries the medium through an aperture 51 inthe tank wall bordering the foam fractionator section 33. The mediumthen passes through the foam fractionators 81 and is carried by acoupling pipe 52 to the carbon filter section 34. After passage throughthe filter substrates 91, the medium enters the filter outlet section 36and is coupled therefrom back to the rearing tanks 4A through 4C by areturn pipe 53.

As above discussed, the particulate filter assembly 37 is formed from aplurality of similar particulate filters 100, each of which receivesmedium to be filtered via the manifold 46 connected to the output of thepump 38. The filters 100 filter particulate matter from the mediumpassing therethrough and operate on a backwash principle. As shown inFIGS. 7 through 9, each of the filters 100 is in the form of acylindrical drum having opposite flat ends 101 comprised of a stiffmaterial such as, for example, plexiglass. A plurality of support ribs102 extend between the drum ends and support a layer of fine mesh net103. The net 103 forms the cylindrical sidewalls of the drum and itsends are also attached to the drum ends. Extending centrally through thedrum and rotatably mounted relative thereto is a conduit 108 whose inputend forms one of the inputs 47 connected to the manifold 46. In theinterior of the drum, a feed pipe 104 having an apertured end 105branches off in a downward direction from the central conduit 108 andcouples medium entering the drum input 47 to the lower portion of thedrum. Slightly downstream from the feed pipe 104, the conduit 108 isblocked by a wall 106 beyond which a further feed pipe 107 extendsupwardly toward the top portion of the drum. The feed pipe 107 couplesfiltered matter to the output end 119 of the conduit 108 which, in turn,is connected to a collector pipe (not shown) for carrying the matter outof the filtration area. The filtered matter is fed to the feed pipe 107from a tray 109 located in the upper portion of the drum and havingslanted side walls which lead to the pipe 107. The tray 109 receivesthis matter from the upper mesh portions of the drum when the matter isdislodged by the action of water sprayed from a spray bar 111 situatedabove the drum and extending along its length. Water is controllably fedto the spray bar 111 through a solenoid valve 112 which is coupled to awater source (not shown).

A float switch 113 supported on the central conduit 108 providesactuation of the solenoid valve 112 as well as a motor 114 provided forrotating the drum relative to the conduit 108 and the tray 109. Themotor 114 drives a first sprocket wheel 115 which, in turn, drives via achain 116, a second sprocket wheel 117 whose hub is connected to thedrum end wall 101.

In operation, when the particulate matter filtered by the interior ofthe lower portion of the mesh of the drum builds up to a point where themedium 15 in the drum rises to a level at which it activates the floatswitch 113, the switch 113 then actuates the motor 114 and solenoidvalve 112. This, in turn, causes the drum to rotate so that a cleansection of mesh is now situated at the lower portion of the drum.Simultaneously, a clogged section of mesh at the upper portion of thedrum is brought under the spray now being delivered from the spray bar111. These actions cause the medium level in the drum to decrease andthe particulate matter dislodged by the spray from the interior of themesh to fall into the tray 109 and be carried out of the system. Thiswill continue until the medium level in the drum decreases to a pointwhere the float switch 113 is no longer activated. At this time, themotor 114 and solenoid 112 turn off and the medium passes through themesh net until the interior of the net becomes sufficiently clogged toallow the medium to build to a level where it again activates the switch113.

The particulate filters 100 thus remove a significant amount ofparticulate debris from the medium 15 being delivered by the pump 34 asthe medium passes downward through the filters into the bio-filtersection 32. As shown, the latter section encompasses a large enclosedarea of the tank 31. This area is filled with a layer of coral rockgravel 71 which is impregnated with nitrification bacteria. Passage ofthe medium 15 through the bio-filter thus results in removal of asignificant amount of metabolic wastes.

As noted previously, below the gravel 71 in the bio-filter area 32 aredisposed apertured pipes 48 for receiving the medium after it has passedthrough the filter. These pipes carry the medium to the collector pipe49 which leads the medium through the aperture 51 in the wall 72separating the foam fractionator section 33 from the bio-filter section32.

As above-indicated, the foam fractionator section 33 includes four foamfractionators 81 each of similar construction, for removing furthercontaminants and, in particular, large organic molecules such asproteins, from the introduced medium from the pipe 49. As shown in FIGS.6, 7, 10 and 11, each fractionator 81 comprises a pipe 82 having helicalindentations 82a along its length and apertures 83 at its upper end forreceiving the introduced medium. A cylindrical collector or funnel 84 issupported centrally within the pipe 82 and extends downwardly past theapertures 83. At the lower end of the pipe 82, an apertured annular bar85 is provided for introducing air to the downwardly flowing medium. Thebar 85 receives air from an air line 86 coupled to an air source (notshown).

As can be appreciated, the medium flowing into the fractionator section33 rises to the height of apertures 83 in the pipes 82 of thefractionators 81, thereby causing medium to descent downwardly througheach pipe. The air introduced from the respective bars 85, in turn,causes bubbles to ascend upwardly through each pipe. These ascendingbubbles interact with the descending medium causing large organicmolecules to be stripped therefrom. The bubbles with the attachedorganic molecules then continue their upward ascent and are collected bythe respective collectors 84. The collected material in the collectorsis then conveyed out of the system by the lines 87 and discarded.

The foam fractionators 81 thus remove further waste material from themedium 15 which, after reaching the bottom of the fractionator section33, is coupled by the pipe 52 to the carbon filter section 34. Thissection includes a plurality of hollow substrates 91 which are filledwith carbon. The hollow substrates 91 are slidably retained for easyremoval and replacement between pairs of vertical channels 92 affixed toopposite walls of the carbon filter section 34. The medium passes fromthe pipe 52 through the substrates 91 and then enters the outlet section36. The return pipe 53 then carries the filtered medium back into therearing tanks 4A to 4C.

As can be appreciated, the combined effect of the particulate filters,bio-filter, foam fractionators and carbon filters of the presentinvention results in a significant amount of contaminants beingextracted from the medium of the rearing tanks. As a result, the qualityof the medium is maintained at a high level, thereby promoting shrimpgrowth and limiting shrimp mortality.

A further cleaning operation can also be carried out to aid thefiltration system in maintaining the medium quality. This cleaningoperation results in cleaning the bottom walls of the rearing tanksalong which debris which cannot be effectively filtered is gathered.More particularly, a conventional automatic pool cleaner, suitablymodified, is inserted into each tank after the substrate units have beenraised from the medium. The robot scrapes the tank bottom and anyaccumulated detritus material is removed and carried out of the rearingbuilding by suitable piping.

In all cases, it is understood that the above-described arrangements aremerely illustrative of the many possible specific embodiments whichrepresent applications of the present invention. Numerous and variedother arrangements can readily be devised in accordance with theprinciples of the present invention without departing from the spiritand scope of the invention.

What is claimed is:
 1. A system for rearing post-larvae shrimpcomprising:an enclosure for receiving a medium; habitat means disposedin said enclosure for providing habitats for said shrimp; and filtrationmeans for continuously filtering the medium in said enclusure, includinga filter assembly and a pump for pumping the medium of said enclosurethrough said filter assembly, said filter assembly including:particulatefilter means connected to the output of said pump; bio-filter means forreceiving the output of said particulate filter means; and foamfractionator means for receiving the output of said bio-filter means. 2.A system in accordance with claim 1 wherein:said system includes anadditional enclosure for receiving said medium and said enclosure andsaid additional enclosure are arranged one after the other and have awall in common; said common wall has an opening for permitting medium toflow from said additional enclosure to said enclosure; and saidenclosure has a further opening for coupling medium to said pump.
 3. Asystem in accordance with claim 1 wherein: said filter assembly furtherincludes:carbon filter means for receiving the output of said foamfractionator means.
 4. A system in accordance with claim 3 wherein: saidfiltration means further includes:a tank disposed adjacent saidenclosure, said tank having walls which partition said tank into first,second and third compartments in which are disposed said bio-filtermeans, said foam fractionator means and said carbon filter means,respectively.
 5. A system in accordance with claim 4 wherein:saidparticulate filter means is supported between the tank walls formingsaid first compartment.
 6. A system in accordance with claim 4wherein:said carbon filter means are slidably retained between oppositewalls forming said third compartment.
 7. A method of rearing post-larvaeshrimp comprising:retaining shrimp within an enclosure filled with amedium; filtering said medium including:passing said medium through afilter assembly comprising: passing said medium through particulatefilter means, passing said medium through bio-filter means comprisingcoral rock gravel impregnated with nitrification bacteria, passing saidmedium through a foam fractionator means, and passing said mediumthrough a carbon filter means; and prior to passing said medium throughsaid filter assembly using a single pump to raise the energy of saidmedium to a level sufficient to enable said medium to pass through saidfilter assembly and return to said enclosure via gravity flow only.